The present invention relates to a multi-beam scanning device which Is configured to form simultaneously a plurality of scanning lines on a surface to be scanned using a plurality of light beams.
Conventionally, an imaging device, such as a laser beam printer, a digital copying machine and the like, which forms an image on a surface to be scanned by scanning a beam spot on the surface, is well known.
Recently, a multi-beam scanning device is provided. The multi-beam scanning device is configured such that a plurality of beam spots scan on the surface to be scanned simultaneously using a single polygonal mirror in order to increase an printing speed substantially.
Japanese Patent Provisional Publication No. HEI 8-304722 discloses such type of a multi-beam scanning device. In HEI 8-304722, a beam splitter is used to change optical paths of a plurality of beams emitted by a light source so that the plurality of beams are parallel with each other and that the plurality of beams are closely adjacent to each other. The parallel beams emerged from the beam splitter are then incident on a single polygonal mirror. Since the multi-beam optical device uses the beam splitter, i.e., a relatively expensive optical component, a cost thereof relatively increases
Japanese Patent Provisional Publication No. 2000-249948 describes another type of the multi-beam scanning device. In this publication, two light beams impinge on a single polygonal mirror at the nearly same position without using the expensive beam splitter. It should be noted that in the multi-beam scanning device in the publication No. 2000-249948, an angle θ is formed between two beams, which are incident on the polygonal mirror, In a plane perpendicular to a rotational axis of the polygonal mirror.
In the multi-beam scanning device described in the publication No. 2000-249948, scanning angular ranges for the two light beams are different from each other, since the angle θ is formed between two incident beams. As a result, in such a case, curvature of fields and/or scanning speeds for the two light beams become different from each other. If such phenomena occur, scanning lines are not formed properly, and therefore, imaging quality is deteriorated.
In addition, if the angle θ is formed between two beams, which are incident on the single polygonal mirror, it becomes necessary to use a polygonal mirror having a relatively wide reflective coating. That is, it becomes necessary to use a polygonal mirror whose diameter of an inscribed circle is relatively large. In such a case, the multi-beam scanning device requires more expensive motor for rotating the polygonal mirror. Therefore, cost of the multi-beam scanning device increases.
Accordingly, a multi-beam scanning device having a small angle θ formed between the two incident beams is desired. In the multi-beam scanning device described in the publication No. 2000-249948, the angle θ between two incident beams depends on diameters of coupling lenses which are respectively placed on optical paths of the two incident beams, and therefore, the angle θ can not be decreased below a certain value determined by the coupling lenses.